The continental United States has an estimated 144 billion barrels of oil resources, of which 68 billion barrels are in the form of heavy oil (63 billion barrels in California), and the rest 76 billion barrels are in the form of natural bitumen contained in tar sand in various states. Utah has the largest tar sand reservoir accounting for almost half of the total in the U.S. Tar sand resources in the U.S. remain virtually undeveloped for a lack of technically and economically viable technologies. Additionally, Canada has the world's largest oil sand reservoir estimated at 4.5 trillion barrels, and nearly half of the oil now used in Canada comes from upgrading oil sand through various conventional methods.
Tar sands are grains of sand or porous rock deposited with bitumen, a very heavy, asphalt-like crude oil. The bitumen must be treated and upgraded before it can be fed to refineries for gasoline and fuel production. Tar sands contain varying fractions of bitumen (10-15% typically) and the remainder of sand, clay, and moisture. Bitumen deposited near the earth's surface can be recovered by open-pit mining techniques, while recovery methods for those deep in earth (>75 m) generally involve pumping and introduction of steam and solvents through vertical or horizontal wells. The mined surface tar sand can be mixed with steam and hot water, which allow the bitumen to float on the water while the sand sinks to the bottom of the container, thus achieving separation. Typical further processing involves heating the bitumen above 500° C. to convert about 70% of it to a synthetic crude oil. This oil can be distilled to yield kerosene and other liquid products. The remainder of the bitumen either thermally cracks to form gaseous products or reacts to form petroleum coke.
Petroleum asphaltenes are hydrocarbons having an extremely complex molecular structure with different proportions of nitrogen, sulfur, and oxygen. Asphaltenes can cause problems such as the blockage of crude oil extraction and transport pipes, and a reduction in their economic use. Asphaltene precipitation has also been a serious problem in oil recovery processes. The increasing production of heavy oils with a high content of asphaltenes makes their processing more difficult. Asphaltenes present in heavy oil and residuum strongly affect upgrading and refining operations. During the hydroprocessing of heavy feedstock, asphaltenes limit the efficiency of conversion and refining, acting as coke precursors that lead to catalyst deactivation.
The effect of asphaltenes and the solubility of asphaltenes in oil on the formation of solids have been extensively studied and reported in the last couple decades. Various approaches have been taken in the upgrading of asphaltenes. Catalytic (Mo) upgrading of Athabasca bitumen vacuum bottoms via a two-step hydrocracking and enhancement of Mo-heavy oil interaction has been recently reported. Solvent processing by dilution and centrifugal separation was explored to remove solids and part of the resin and asphaltene prior to hydrocracking. Further, reforming reactions of bitumen was carried out with kerosene as a solvent under nitrogen atmosphere. Toe-to-heel air injection was developed to enable high oil recovery and substantial in situ upgrading. The (HC)3TM hydrocracking technology was used to upgrade heavy crude oils and residues that was based on a hydrocracking process that used a two-phase, gas-liquid slurry reactor to convert up to 95% by weight of heavy oils and residues into distillates. Hydrotreatment of asphaltenes in the crude oil was shown to result in a complete conversion of the asphaltenes. Upgrading of heavy crude was carried out using either a hydrogen donor, formic acid as hydrogen precursor, or hydrogen donors and methane under steam injection conditions. Heavy oil upgrading process was carried out first by thermal cracking using visbreaking or hydrovisbreaking technologies to produce products with lower molecular weights and boiling points, followed by deasphalting and separation using an alkane solvent.
Other upgrade techniques for asphaltenes include: thermal cracking, thermal cracking in different solvents, solvent deasphalting followed by slurry hydroprocessing, through ultrasonic cavitation and surfactant use, superacid-catalyzed hydrocracking, superacid-catalyzed hydrocracking with supercritical water, hydrogenation with a NiMo-supported catalyst, ultrafiltration using ceramic membrane, using deasphalted oil, supercritical fluid extraction, thermal hydroprocessing, using a graded mesoporous catalyst system, sonochemical treatment, hot water treatment containing carbonate, hydropyrolysis, hydrocracking with an Mo-additive, hydrocracking with an Mo-additive, water, and transition metal catalysts, biocatalytic transformation through a modified cytochrome C, and flash-coking with the Lurgi-Ruhrgas process and hydrotreating.
The use of ozone in upgrading bitumen has sporadically appeared in the literature over the years. Ozone has been used in conversion of bitumen in tar sand into water-soluble derivatives. The process involved treatment of the tar sand with oxygen, air, or ozone and subsequently with an alkali sulfite solution. The resulting water-soluble sulfonated bituminous derivatives have emulsifying and dispersing power that may have potential for the in situ extraction of bitumen from tar sand. Ozone has been applied in the thermal dissolution of Kangalassy brown coal in tetralin, which resulted in increased solution due to change of oxygen-containing group properties of the product fluid. Tadzhikistan petroleum asphaltenes were ozonated producing C6-18 mono- and dicarboxylic acids as principal ether-soluble products, with small amounts of dicyclonaphthenes, keto-, hydroxyl-, and alkoxy acids, along with 0.03-0.4 wt. % S and N. Additionally, demulsifier products were obtained by ozonation in CCl4/MeOH at a 1:(0.5-1) volume ratio followed by boiling with an alcohol solution of alkali.
Upgrading or extracting bitumen in supercritical fluids, e.g., water, ethane, CO2, have been attempted. Upgrading of bitumen by hydrothermal visbreaking in supercritical water with alkali has produced paraffins and aromatics similar to pyrolysis, with similar time dependences of the visbreaking and desulfurization reactions but different effects of water. Upgrading of asphaltene by supercritical water with and without partial oxidation has been studied. The extraction of Peace River bitumen using supercritical ethane produced varying product properties with respect to conditions and time of extraction. Upgrading of bitumen using various n-alkanes as solvents in supercritical fluids has been performed. The process used activated carbon catalysts in a bench-scale plug-flow reactor, which produced favorable results when significant SCF, highly saturated or paraffinic supercritical solvent, hydrogen, and activated carbon catalyst were present. Other solvents, including pentane/benzene, CO2, propane, pentane, were also used in supercritical conditions to extract bitumen from tar sand.
With the world-wide increase in the demand for oil and related products, devices and methods for improved recovery and upgrading of heavy hydrocarbons continue to be sought.